Bat Echolocation Researc h - Bat Conservation International

The chance of detection also depends on the nature
of the habitat and amount of clutter present. In an open
field, any system will sample a cone-shaped volume with
a relatively even detection probability within this cone.
At a forest edge or on a forest path, the shape of the cone
will be biased towards the open part of the habitat (Fig.
6). The directionality of a microphone is also frequency
dependent, with more directionality at higher frequencies
than at lower frequencies. In wide-angle microphones,
frequency will have more impact than in directional
microphones. So at least theoretically, we may
expect differences in sampled area, and in detection
chance between species in relation to the frequencies
they use. The sound produced by the bats also varies in
its directionality.
In studies whose purpose is to make detailed observations
of flight course and flight behavior, a double array
of microphones has been used to accurately locate the bat
(Holderied 2001). The exact position can then be calculated
by triangulating, pulse by pulse, signals from the different
microphones. Wide-angle microphones will
enhance the chance of detecting the bat in all microphones.
However, as wide-angle microphones are generally
less sensitive, the range is reduced (Holderied 2001).
Table 2: A comparison of heterodyne, frequency-division,
and time-expansion detection and conversion systems with
respect to the potential these systems provide for interpretation
of audible parameters for identification in the field and/or
the analysis of recorded sound involving oscillograms, multispectrograms,
frequency spectrograms, and zero crossing.
Legend: ++ = good, + = possible, +/- = difficult, - = not possible,
or: ++ = relatively inexpensive, + = intermediate, +/- = relatively expensive.
* no direct information, but information on FR rate/shape/curvature can be
deduced from tonal quality, change of pitch while tuning, position of QCF,
or shallow FM in the pulse relative to steeper FM.
Section 2: Acoustic Inventories
Broadband versus Narrowband Detectors
Broadband systems have the advantage of allowing
simultaneous detection of different bats that use different
frequencies. Although TE and FD systems are broadband,
theoretically enabling observers to monitor the
entire bandwidth used by the bats, limitations are set by
the frequency response of the microphones and the filters
that are incorporated into the detector to reduce
noise at lower frequencies.
Tuned heterodyning systems optimally detect signals
in the narrow frequency window around the tuned frequency.
This is an advantage in various situations. It
allows the researcher to concentrate on particular
species outside the frequency range of others. Tuning to
the loudest frequency can also increase detection range.
The detector can be tuned outside potential sources of
noise, such as traffic or insects. Conversely, bats using
frequencies outside of the tuned frequency range will be
missed (Fig. 7). With a handheld detector, this can be
compensated for by manually scanning the frequencies
of interest. For stationary detectors, a ‘best setting’ has to
be chosen, where all or most of the expected species will
be detected. In some models of heterodyne systems, an
automated scanning function is available.
THE DETECTOR AS A TOOL FOR IDENTIFICATION
Many current applications of bat detectors concern
linking species to their calls for species identification.
These studies started with high-frequency microphones
connected to instrumentation tape recorders (e.g., Ahlén
1981, 1990; Fenton and Bell 1981), and advanced to
recording of frequency-division signals (Ahlén 1981,
1990; Ahlén et al. 1984; De Jong and Limpens 1985;
Weid and Von Helversen 1987), to recording timeexpanded
signals on DAT recorders or direct recording
Figure 6: Nature of sampling bias as a function of habitat complexity.
31